The present invention relates to stored charge circuits having transient error, and more particularly, to active bandpass filter circuits having a ringing response or the like to a step function or other transient error initiating input signal.
It is well known in the prior art that the transient response of circuits having reactive components have a transient error in response to sharply changing input signals such as step functions, pulses, square waves, or the like. For example, in a very simple application, an RC time constant charging circuit can be designed to exhibit a critically damped response in which case the output has a single overshoot of predetermined amplitude but quick response time. Another example is an overly damped response where there is no overshoot response but the response time for the output of the circuitry to acquire the quiescent condition is much longer than that for a critically damped response. A further example is an underdamped response where there is a ringing about the long term quiescent output value with the sequential amplitudes decreasing with time. Similar transient error effects are exhibited by passive RC networks connected as low pass, high pass, bandpass filters, or ladder networks.
In active circuits, this transient error is particularly troublesome in feedback circuits having a plurality of complex poles such as in active filter circuits and feedback control systems of servo mechanisms where a ringing or an underdamped response will cause the mechanism to hunt. This hunting characteristic, due to the transient error response of the feedback servo control circuits, often presents a major design problem affecting the response time of the servo mechanism and the accuracy with which the servo mechanism will follow the control signal.
Such a transient error response is usually attributable to delay in capacitors of stored charge circuits from achieving their final charge or voltage due to associated resistances in the circuitry. If the capacitors of the circuit could achieve their steady state charge voltage as determined by the input signal without inhibition from the circuit resistances, the transient error of the circuit would be significantly reduced and the circuit would be more rapidly reset to enable the circuit to respond to the next closely time spaced signal without any overhang interference from the preceeding signal. Accordingly, it is desirable to reduce the transient error of stored charge circuits in response to step functions and other fast rise time input signals.